Martin Richard, the 8-year-old boy killed in the Boston Marathon bomb attack, once held a sign that said 'peace' and 'no more hurting people.' Research finds a pattern of lessening violence as human history moves forward.

A photo of Martin Richard, 8, hangs at a makeshift memorial near the finish line of Monday's Boston Marathon explosions, which killed three and injured more than 170. Martin was killed in the explosions.

Of all the images from the Boston Marathon tragedy that became suddenly iconic this week, none was more poignant than this: a photo showing how Martin Richard, the 8-year-old boy who died in the bomb attack, once held a sign that said “peace” and “no more hurting people.”

As a city and nation struggle to move forward, that sign implies one of the big questions that remains: What can be done to prevent such acts of senseless destruction in the future?

In the case of the explosions that rocked the Boston Marathon finish line on April 15, the suspects identified Thursday by the FBI have now been captured. Tamerlan Tsarnaev died in a gun battle with police, while his younger brother, Dzhokhar Tsarnaev, is at a Boston hospital under tight security.

But their motive was still a matter of investigation Saturday.

Whatever is eventually determined – whether the attacks stemmed from affiliation with some terrorist ideology or by something else – officials in cities around the nation are now thinking harder about how to protect against such potential attacks, notably on “soft targets” like an outdoor road race that are difficult to secure.

One answer, already, is stepped-up security measures by law enforcement.

From public events in America this weekend to the running of the London Marathon this Sunday, the presence of law enforcement is greater than it would have been had the Boston attack not occurred. But, although Boston in recent days has seen a dramatic “surge” of police and National Guard troops, limited government budgets and the huge number of soft targets mean that such efforts are an imperfect defense.

Another part of the answer is public vigilance – ordinary people being alert about behavior that raises doubts about the intentions or mental stability of acquaintances. Again, this is an imperfect defense.

Some people who knew 19-year-old Dzhokhar Tsarnaev in school said that he did normal activities like sports and parties. “He was never a troublemaker,” one former teacher said.

The Boston case also coincides with growing public debate about three issues with big implications for prevention of violent crime and terrorism: Gun control, immigration reform, and civil liberties in an era of drones and databases of online information.

On firearms, this was a case where the alleged bombers used guns as well as explosives. One of four people killed in the bombing and its aftermath was an MIT campus police officer who was shot while in his car.

The Tsarnaev brothers exchanged gunfire with police during a chase and manhunt that ended Friday night.

This comes during a week when supporters of stronger background checks for gun purchases failed in a US Senate vote. The National Rifle Association and some others argue that Americans’ safety can be enhanced through a greater presence of armed “good guys,” including guards to prevent Newtown-style tragedies in schools. At the same time, many Americans want to see access to assault weapons restricted, and efforts to ensure that people with criminal records or diagnosed mental disorders can’t buy firearms.

On immigration, this case connects in the sense that the brothers where of Chechen heritage, born in Russia and Kyrgyzstan. Teenage Dzhokhar Tsarnaev became a US citizen last year, while his older brother Tamerlan, 26, was in the US on a green card.

The New York Times reported that Tamerlan was interviewed by federal officials within the past couple of years, when a foreign government asked the FBI if he had extremist ties. The foreign government worried that Tsarnaev might be a security threat if he visited that nation on a planned trip, the Times reported, citing an unnamed government official.

Amid efforts to determine whether this or other facts about the Tsarnaevs should have been “red flags” that brought closer scrutiny from immigration or FBI authorities, at the very least the bombings serve as a reminder that debate over immigration reform includes a public-safety element.

Although civil liberties aren’t in the public spotlight to quite the same degree as guns or immigration, the Boston case also comes as America is wrestling with the right balance between defending freedoms and seeking to ensure public safety.

And again, the case brings some of these issues to the surface. One example: It shows how technology that diminishes personal privacy can also aid in making an arrest.

Footage from a Lord & Taylor surveillance camera proved essential in identifying the Tsarnaevs as the bombing suspects. Instances such as that could prompt efforts to boost the volume and quality of video-camera technology in public spaces across the US.

And with the ties of the Tsarnaev family to Chechnya, a hotbed of Muslim radicalization after years of rebellion against Russia’s central government, some observers have cautioned this week against the temptation to engage in “profiling” people as suspicious because of their ethnic or religious background.

This risk was highlighted, in an almost humorous way, as the ambassador to the US from the Czech Republic felt impelled to issue a public statement, saying, “I am concerned to note in the social media a most unfortunate misunderstanding…. The Czech Republic and Chechnya are two very different entities – the Czech Republic is a Central European country; Chechnya is a part of the Russian Federation.”

Securing the nation from future attacks, judicial experts say, involves preserving the right balance between efforts at vigilance – including tips from the public – and guarding the bonds of liberty and mutual trust that hold a diverse “nation of immigrants” and descendants of immigrants together.

“America is not organized the way countries are usually organized,” he said. “We are not organized around a common language or religion or even culture. We are organized around a handful of civic ideals” of equality, opportunity, freedom, and fair play. “We must not permit darkness and hate to triumph over our civic faith.”

No one is expecting that such ideals, coupled with heightened vigilance, will suddenly make the dream of perfect peace – young Martin Richard’s expression – a reality.

But it’s worth noting that some recent academic research, done right near Boston by Harvard University’s Steven Pinker, finds a pattern of lessening violence as human history moves forward.